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plurals

POcheck if element exists on the sub-list
primarykey
Id
5755164
data
AcceptedAnswerId
5755462
AnswerCount
2
ClosedDate
CommentCount
0
CommunityOwnedDate
CreationDate
2011-04-22T11:59:53.970
FavoriteCount
0
LastActivityDate
2011-04-22T12:37:21.767
LastEditDate
2011-04-22T12:23:01.400
LastEditorUserId
495796
OwnerUserId
711297
ParentId
0
PostTypeId
1
Score
1
ViewCount
782
LastEditorDisplayName
text
Body
<p>I can have many Figures on my list. Each Figure can have many Rectangles on its list. I have a problem with my function checkNewRectangleId - this function should ask user about new rectangle id until he write really new id and then it should return this id - but I have an error: couldn't match expected type IO t against inferred type Maybe figureType line <code>(Figure id width height rectangles) <- findFigure idFigure x</code> in my function - could you help ?</p> <pre><code>import IO import Char import System.Exit import Maybe import Data.Time.Calendar import System.Time checkNewRectangleId :: Int -> [FigureType] -> IO Int checkNewRectangleId idFigure x = do idRectangle <- getInt "Give me new rectangle id: " (Figure id width height rectangles) <- findFigure idFigure x if isJust (findRectangle idRectangle rectangles) then do putStrLn ("We have yet rectangle with id " ++ show idRectangle) checkNewRectangleId idFigure x else return idRectangle data FigureType = Figure Int Int Int [RectangleType] deriving(Show, Read) data RectangleType = Rectangle Int CalendarTime deriving(Show, Read) findFigure :: Int -> [FigureType] -> Maybe FigureType findFigure _ [] = Nothing findFigure n ((Figure id width height rectangles) : xs) = if n == id then Just (Figure id width height rectangles) else findFigure n xs findRectangle :: Int -> [RectangleType] -> Maybe RectangleType findRectangle _ [] = Nothing findRectangle n ((Rectangle id date) : xs) = if n == id then Just (Rectangle id date) else findRectangle n xs isInt i = not (null i) && all isDigit i getInt :: String -> IO Int getInt q = do putStr q; i <- getLine if isInt i == False then do putStrLn "Bad number" getInt q else return (read i) </code></pre>
Tags
<haskell><monads><type-mismatch>
Title
check if element exists on the sub-list
singulars
PostAcceptedAnswerId
1. PO
  singulars
  PostTypePostTypeId
  PTAnswer
PostParentId
1. This table or related slice is empty.
PostTypePostTypeId
1. PTQuestion
UserLastEditorUserId
1. USRobin Green
UserOwnerUserId
1. USmrquestion
plurals
PostLinksPostIdRelatedPostId
1. This table or related slice is empty.
PostLinksRelatedPostIdPostId
1. This table or related slice is empty.
PostsAcceptedAnswerId
1. This table or related slice is empty.
PostsParentIdCreationDate
1. PO
  singulars
  PostTypePostTypeId
  PTAnswer
2. PO
  singulars
  PostTypePostTypeId
  PTAnswer
VotesPostIdCreationDate
1. VO
  singulars
  PostPostId
  POcheck if element exists on the sub-list
  UserUserId
  This table or related slice is empty.
  VoteTypeVoteTypeId
  VTUpMod
CommentsPostId
1. This table or related slice is empty.

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